The present invention relates to a method and apparatus for non-destructively monitoring a specimen for density variations. More specifically, it relates to the application of certain phenomena and principles of gamma-ray-transmission techniques to the generating, detecting, and interpreting of positron-annihilation-radiation incident upon, and interacting with, and attenuated by the interrogated specimen.
In many applications, it is necessary to discern the presence of adjacent materials of differing densities, otherwise obscured from view. For example, it may be desirable to detect the presence within an opaque vessel or structure, of heavy fluids, such as water, oil, liquid metals, etc., located in cavities, channels, voids, or galleries normally invisible, or otherwise inaccessible, to probes.
A specific example of such an application deals with the maintenance and reliability of aircraft. It is well known that dynamic balance of an aircraft propellor must be achieved in order that vibration problems might minimized. Quite often, if vibrations are sensed in an aircraft, and if propellor imbalance is deemed blameworthy, the propellors must be removed and disassembled in order that the reason(s) for propellor-blade imbalance might be learned. In such instances, oil from the pitch-control mechanism located in the propellor hub might be found to have infiltrated the plugged cavity in the blade shank, causing imbalance. Such examinations are time-consuming, at best, when invasive oil is indeed found, and are particularly wasteful of time in those cases in which the cavity proves to be empty.
The utilization of conventional X-ray radiography techniques to sense the presence of oil in the blade cavity has several drawbacks. In particular, it requires expensive x-ray equipment, configured specifically for the intended application, which would have to be located at each installation where at the aircraft are based. Moreover, specially-trained personnel are required to operate and maintain X-ray equipment. And, finally, monitoring, by radiation-safety personnel, of the X-ray equipment might be inconvenient, as well as impractical.
An optimal densitometer would have several basic capabilities. First, it should not present a health hazard to personnel utilizing the device. It should utilize a readily available, encapsulated, long-lived radioactive positron-emitting source, its strength not exceeding ten microCuries, thereby obviating radiation hazard, and eliminating, hence, the need for licensing the source, or parts of the apparatus, or its application procedures.
Second, it should be versatile and reliable. It should be portable to accommodate measurements "in the field", exhibit a high degree of spatial resolution, and operate over a wide dynamic range.
And, finally, it should be relatively insensitive to normal levels of environmental background radiation.
It is believed that prior to the present invention, there has not been available, conforming with the constraints and meeting the requirements cited above, a method and apparatus for nondestructively measuring the spatial variation of density within an opaque vessel or structure. Thus, the need for such a system had heretofore gone unfulfilled.
It is accordingly a general object of the present invention to overcome the aforementioned limitations and drawbacks associated with conventional densitometers, and to fulfill the needs mentioned, by providing a method and apparatus for sample density monitoring having all of the desirable attributes noted above.
It is a particular object of the invention to provide a radiographic method and apparatus for nondestructively monitoring a specimen for density variations.
It is a further object of the invention to provide a method and apparatus predicated upon positron-annihilation-radiation interactions for nondestructively ascertaining abrupt, as well as gradual, spatial variation of density within a specimen.
Other objects will be apparent in the following detailed description, and in the practice of the invention.